Cathode material for lithium-ion secondary battery, cathode for lithium-ion secondary battery, and lithium-ion secondary battery

ABSTRACT

A cathode material for a lithium-ion secondary battery of the present invention is active material particles including central particles represented by Li x A y M z PO 4  and a carbonaceous film that coats surfaces of the central particles, in which a median diameter is 0.50 μm or more and 0.80 μm or less, and a chromaticity b* in an L*a*b* color space is 1.9 or more and 2.3 or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Patent Application No.2016-192877 filed Sep. 30, 2016, the disclosure of which is hereinincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a cathode material for a lithium-ionsecondary battery, a cathode for a lithium-ion secondary battery, and alithium-ion secondary battery.

Description of Related Art

In recent years, as batteries anticipated to have a small size and ahigh capacity and weigh less, non-aqueous electrolytic solution-basedsecondary batteries such as lithium-ion secondary batteries have beenproposed and put into practical use. Lithium-ion secondary batteries areconstituted of a cathode and an anode which have properties capable ofreversibly intercalating and deintercalating lithium ions, and anon-aqueous electrolyte.

As anode active materials for anode materials of lithium-ion secondarybatteries, generally, carbon-based materials or Li-containing metaloxides having properties capable of reversibly intercalating anddeintercalating lithium ions are used. Examples of the Li-containingmetal oxides include lithium titanate (Li₄Ti₅O₁₂).

Meanwhile, as cathodes of lithium-ion secondary batteries, cathodematerial mixtures including a cathode material, a binder, and the likeare used. As the cathode active material, for example, Li-containingmetal oxides having properties capable of reversibly intercalating anddeintercalating lithium ions such as lithium iron phosphate (LiFePO₄)are used. In addition, cathodes of lithium-ion secondary batteries areformed by applying the cathode material mixture onto the surface of ametal foil that is called an electrode current collector.

As electrolytic solutions for lithium-ion secondary batteries,non-aqueous solvents are used. Non-aqueous solvents enable theapplication of cathode active materials that are oxidized and reduced ata high potential or anode active materials that are oxidized and reducedat a low potential. Therefore, lithium-ion secondary batteries having ahigher voltage can be realized.

These lithium-ion secondary batteries have a small size and a higherenergy and weigh less than secondary batteries in the related art suchas lead batteries, nickel cadmium batteries, and nickel metal hydridebatteries. Therefore, lithium-ion secondary batteries are used not onlyas small-size power supplies used in portable electronic devices such asmobile phones and notebook personal computers but also as large-sizestationary emergency power supplies.

In recent years, there has been a demand for the performance improvementof lithium-ion secondary batteries, and a variety of studies have beencarried out. For example, in a case in which a lithium-ion secondarybattery is used in a high-current density region, there is a demand foradditional improvement in electron conductivity in order to improve theperformance. Regarding the above-described property demands, techniquesfor coating the surfaces of cathode active materials with a carbonaceousmaterial (hereinafter, in some cases, referred to as “carbonaceousfilm”) are known (for example, refer to Japanese Laid-open PatentPublication No. 2009-004371, Japanese Laid-open Patent Publication No.2011-049161, and Japanese Laid-open Patent Publication No. 2012-104290).As a method for coating the surface of a cathode active material with acarbonaceous film, methods in which a cathode active material and acarbon source are mixed together and this mixture is calcinated in aninert atmosphere or a reducing atmosphere are known.

SUMMARY OF THE INVENTION

When the cathode density is improved by filling the carbonaceous filmthat coats the surface of the cathode active material with a largeramount of a cathode active material, the energy density per unit volumeimproves. However, in a case in which the cathode active materialagglomerates and thus forms an agglomerate, voids are generated amongthe cathode active material grains in the carbonaceous film, and thusthere is a problem in that the cathode density decreases. Therefore,techniques in which the agglomerate made of the cathode active materialis cracked in a gas phase or the like so as to decrease the agglomeratediameter, and voids among the cathode active material grains areefficiently filled with the cathode active material, thereby improvingthe energy density per unit volume is known.

However, in a case in which a cathode material including the primaryparticles of a cathode active material such as lithium iron phosphateand a carbonaceous film that coats the surfaces of the primary particlesis cracked, when the cracking intensity is too high, the carbonaceousfilm is peeled off from the surfaces of the primary particles, and thusthe electron conductivity decreases. A decrease in the electronconductivity causes the deterioration of input and outputcharacteristics or a decrease in the capacity after a charge anddischarge cycle, which is not preferable. In addition, when the crackingintensity is too low, the cathode density decrease, which is notpreferable.

For the above-described reasons, it is most preferable to crack thecathode material to necessary agglomerate grain sizes while guaranteeingelectron conductivity so as to prevent the carbonaceous film from beingpeeled off. However, the cracking intensity at which the carbonaceousfilm begins to be peeled off varies depending on the primary particlediameter of the cathode active material, the amount of carbon includedin the cathode material, or the like, and thus it is difficult tooptimally control the cracking intensity.

The present invention has been made in consideration of theabove-described circumstances, and an object of the present invention isto provide a cathode material for a lithium-ion secondary battery whichsuppresses the peeling of a carbonaceous film that coats the surfaces ofprimary particles of a cathode active material and is capable ofimproving the cathode density while guaranteeing the electronconductivity, a cathode for a lithium-ion secondary battery includingthe cathode material for a lithium-ion secondary battery, and alithium-ion secondary battery including the cathode for a lithium-ionsecondary battery.

The present inventors and the like carried out intensive studies inorder to achieve the above-described object, consequently found that,when, in active material particles including central particlesrepresented by Li_(x)A_(y)M_(z)PO₄ (0.95≤x≤1.1, 0.8≤y≤1.1, and 0≤z≤0.2;here, A represents at least one element selected from the groupconsisting of Fe, Mn, and Ni, and M represents at least one elementselected from the group consisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al,Ga, In, Si, Ge, and rare earth elements) and a carbonaceous film thatcoats surfaces of the central particles, the median diameter is 0.50 μmor more and 0.80 μm or less, and the chromaticity b* in the L*a*b* colorspace is 1.9 or more and 2.3 or less, it is possible to provide acathode material for a lithium-ion secondary battery which suppressesthe peeling of the carbonaceous film that coats the surfaces of theprimary particles of a cathode active material and is capable ofimproving the cathode density while guaranteeing the electronconductivity, and completed the present invention.

A cathode material for a lithium-ion secondary battery of the presentinvention is active material particles including central particlesrepresented by Li_(x)A_(y)M_(z)PO₄ (0.95≤x≤1.1, 0.8≤y≤1.1, and 0≤z≤0.2;here, A represents at least one element selected from the groupconsisting of Fe, Mn, and Ni, and M represents at least one elementselected from the group consisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al,Ga, In, Si, Ge, and rare earth elements) and a carbonaceous film thatcoats surfaces of the central particles, in which a median diameter is0.50 μm or more and 0.80 μm or less, and a chromaticity b* in an L*a*b*color space is 1.9 or more and 2.3 or less.

A cathode for a lithium-ion secondary battery of the present inventionis a cathode material for a lithium-ion secondary battery including anelectrode current collector and a cathode mixture layer formed on theelectrode current collector, in which the cathode mixture layer includesthe cathode material for a lithium-ion secondary battery of the presentinvention.

A lithium-ion secondary battery of the present invention includes thecathode for a lithium-ion secondary battery of the present invention.

According to the cathode material for a lithium-ion secondary battery ofthe present invention, since the median diameter is 0.50 μm or more and0.80 μm or less, and the chromaticity b* in the L*a*b* color space is1.9 or more and 2.3 or less, it is possible to provide a cathodematerial for a lithium-ion secondary battery which suppresses thepeeling of the carbonaceous film that coats the surfaces of the primaryparticles of a cathode active material and is capable of improving thecathode density while guaranteeing the electron conductivity.

According to the cathode for a lithium-ion secondary battery of thepresent invention, since the cathode material for a lithium-ionsecondary battery of the present invention is included, a lithium-ionsecondary battery having a high energy density and excellent input andoutput characteristics can be obtained.

According to the lithium-ion secondary battery of the present invention,since the cathode for a lithium-ion secondary battery of the presentinvention is included, a lithium-ion secondary battery having a highenergy density and excellent input and output characteristics can beobtained.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of a cathode material for a lithium-ion secondary battery, acathode for a lithium-ion secondary battery, and a lithium-ion secondarybattery of the present invention will be described.

Meanwhile, the present embodiment is specific description for betterunderstanding of the gist of the invention and does not limit thepresent invention unless particularly otherwise described.

Cathode Material for Lithium-Ion Secondary Battery

A cathode material for a lithium-ion secondary battery of the presentembodiment (hereinafter, in some cases, simply referred to as “cathodematerial”) is active material particles including central particlesrepresented by Li_(x)A_(y)M_(z)PO₄ (0.95≤x≤1.1, 0.8≤y≤1.1, and 0≤z≤0.2;here, A represents at least one element selected from the groupconsisting of Fe, Mn, and Ni, and M represents at least one elementselected from the group consisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al,Ga, In, Si, Ge, and rare earth elements) and a carbonaceous film thatcoats surfaces of the central particles, in which the median diameter is0.50 μm or more and 0.80 μm or less, and the chromaticity b* in theL*a*b* color space is 1.9 or more and 2.3 or less.

The average primary particle diameter of the cathode material for alithium-ion secondary battery (active material particles) of the presentembodiment is preferably 10 nm or more and 700 nm or less and morepreferably 20 nm or more and 500 nm or less.

When the average primary particle diameter of the cathode material is 10nm or more, the specific surface area of the cathode material increases,and thus an increase in the mass of necessary carbon is suppressed, andit is possible to suppress a decrease in the charge and dischargecapacity of lithium-ion secondary batteries. On the other hand, when theaverage primary particle diameter of the cathode material is 700 nm orless, it is possible to suppress the extension of the time taken forlithium ions or electrons to migrate in the cathode material. Therefore,it is possible to suppress the deterioration of the outputcharacteristics attributed to an increase in the internal resistance oflithium-ion secondary batteries.

Here, the average particle diameter refers to the volume-averageparticle diameter. The average primary particle diameter of the primaryparticles of the cathode material can be measured using a laserdiffraction and scattering particle size distribution measurementinstrument or the like. In addition, it is also possible to arbitrarilyselect a plurality of primary particles observed using a scanningelectron microscope (SEM) and compute the average particle diameter ofthe primary particles.

The amount of carbon included in the cathode material for a lithium-ionsecondary battery, that is, the amount of carbon that forms thecarbonaceous film is preferably 0.1 parts by mass or more and 10 partsby mass or less and more preferably 0.6 parts by mass or more and 3parts by mass or less with respect to 100 parts by mass of the centralparticles.

When the amount of carbon is 0.1 parts by mass or more, the dischargecapacity of lithium-ion secondary batteries at a high charge-dischargerate increases, and it is possible to realize sufficient charge anddischarge rate performance. On the other hand, when the amount of carbonis 10 parts by mass or less, it is possible to suppress the batterycapacity of lithium-ion secondary batteries per unit mass of the cathodematerial being decreased more than necessary.

The carbon supporting amount with respect to the specific surface areaof the primary particles of the central particles constituting thecathode material for a lithium-ion secondary battery (“[the carbonsupporting amount]/[the specific surface area of the primary particlesof the central particles]”; hereinafter, referred to as “the carbonsupporting amount ratio”) is preferably 0.01 g/m² or more and 0.5 g/m²or less and more preferably 0.03 g/m² or more and 0.3 g/m² or less.

When the carbon supporting amount ratio is 0.01 g/m² or more, thedischarge capacity of lithium-ion secondary batteries at a highcharge-discharge rate increases, and it is possible to realizesufficient charge and discharge rate performance. On the other hand,when the carbon supporting amount ratio is 0.5 g/m² or less, it ispossible to suppress the battery capacity of lithium-ion secondarybatteries per unit mass of the cathode material being decreased morethan necessary.

The BET specific surface area of the cathode material for a lithium-ionsecondary battery is preferably 5 m²/g or more and 25 m²/g or less.

When the BET specific surface area is 5 m²/g or more, the coarsening ofthe cathode material is suppressed, and thus it is possible to increasethe diffusion rate of lithium ions among the particles. Therefore, it ispossible to improve the battery characteristics of lithium-ion secondarybatteries. On the other hand, when the BET specific surface area is 25m²/g or less, it is possible to increase the cathode density in cathodesincluding the cathode material for a lithium-ion secondary battery ofthe present embodiment, and thus it is possible to provide lithium-ionsecondary batteries having a high energy density.

The median diameter of the cathode material for a lithium-ion secondarybattery is 0.50 μm or more and 0.80 μm or less and preferably 0.55 μm ormore and 0.75 μm or less.

When the median diameter is 0.50 μm or more, it is possible to preventelectron conductivity from being decreased due to excessive cracking. Onthe other hand, when the median diameter is 0.80 μm or less, it becomespossible to densely fill cathodes with the cathode active materialduring the production of the cathodes including the cathode material fora lithium-ion secondary battery, and the energy density per unit volumeimproves.

The chromaticity b* in the L*a*b* color space of the cathode materialfor a lithium-ion secondary battery is 1.9 or more and 2.3 or less andpreferably 1.95 or more and 2.3 or less.

The chromaticity b* of the cathode material for a lithium-ion secondarybattery is an index indicating the degree of coating of the carbonaceousfilm with the central particles.

When the chromaticity b* is 1.9 or more, it becomes possible to denselyfill cathodes with the cathode active material during the production ofthe cathodes including the cathode material for a lithium-ion secondarybattery, and the energy density per unit volume improves. On the otherhand, when the chromaticity b* is 2.3 or less, in the cathode materialfor a lithium-ion secondary battery, it is possible to set the degree ofexposure of central particles that are not coated with the carbonaceousfilm in a range sufficient enough to increase the energy density perunit volume, and it is possible to prevent electron conductivity frombeing decreased due to excessive cracking.

Central Particles

The central particles constituting the cathode material for alithium-ion secondary battery of the present embodiment are made of acathode active material represented by Li_(x)A_(y)M_(z)PO₄ (0.95≤x≤1.1,0.8≤y≤1.1, and 0≤z≤0.2; here, A represents at least one element selectedfrom the group consisting of Fe, Mn, and Ni, and M represents at leastone element selected from the group consisting of Mg, Ca, Co, Sr, Ba,Ti, Zn, B, Al, Ga, In, Si, Ge, and rare earth elements).

Meanwhile, the rare earth elements refer to 15 elements of La, Ce, Pr,Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu which belong to thelanthanum series.

The average primary particle diameter of the primary particles of thecentral particles constituting the cathode material particles for alithium-ion secondary battery of the present embodiment is preferably 5nm or more and 800 nm or less and more preferably 20 nm or more and 500nm or less.

When the average primary particle diameter of the primary particles ofthe central particles is 5 nm or more, it is possible to sufficientlycoat the surfaces of the primary particles of the central particles withthe carbonaceous film. In addition, it is possible to increase thedischarge capacity of lithium-ion secondary batteries during high-speedcharge and discharge and realize sufficient charge and dischargeperformance. On the other hand, when the average primary particlediameter of the primary particles of the central particles is 800 nm orless, it is possible to decrease the internal resistance of the primaryparticles of the central particles. In addition, it is possible toincrease the discharge capacity of lithium-ion secondary batteriesduring high-speed charge and discharge.

The shape of the primary particles of the central particles constitutingthe cathode material for a lithium-ion secondary battery of the presentembodiment is not particularly limited, but the shape of the primaryparticles of the central particles is preferably a spherical shape sinceit is easy to generate the cathode active material made of a spherical,particularly, truly spherical agglomerate.

When the shape of the primary particles of the central particles is aspherical shape, it is possible to decrease the amount of a solvent whencathode material paste is prepared by mixing the cathode material for alithium-ion secondary battery, a binder resin (binding agent), and asolvent. Furthermore, when the shape of the primary particles of thecentral particles is a spherical shape, it becomes easy to apply thecathode material paste to electrode current collectors. Furthermore,when the shape of the primary particles of the central particles is aspherical shape, the surface area of the primary particles of thecentral particles is minimized, and thus it is possible to minimize theamount of the binder resin (binding agent) blended into the cathodematerial paste. As a result, it is possible to decrease the internalresistance of cathodes for which the cathode material for a lithium-ionsecondary battery of the present embodiment is used. In addition, whenthe shape of the primary particles of the central particles is aspherical shape, it becomes easy to closely pack the cathode material,and thus the amount of the cathode material for a lithium-ion secondarybattery packed per unit volume of the cathode increases. As a result, itis possible to increase the cathode density, and high-capacitylithium-ion secondary batteries can be obtained.

Carbonaceous Film

The carbonaceous film coats the surfaces of the central particles.

When the surfaces of the central particles are coated with thecarbonaceous film, it is possible to improve the electron conductivityof the cathode material for a lithium-ion secondary battery.

The thickness of the carbonaceous film is preferably 0.2 nm or more and10 nm or less and more preferably 0.5 nm or more and 4 nm or less.

When the thickness of the carbonaceous film is 0.2 nm or more, it ispossible to prevent the excessively thin thickness of the carbonaceousfilm from disabling the formation of films having a desired resistancevalue. In addition, it is possible to ensure conductive propertiessuitable for the cathode material for a lithium-ion secondary battery.On the other hand, when the thickness of the carbonaceous film is 10 nmor less, it is possible to suppress a decrease in the battery capacityper unit mass of the cathode material for a lithium-ion secondarybattery.

In addition, when the thickness of the carbonaceous film is 0.2 nm ormore and 10 nm or less, it becomes easy to closely pack the cathodematerial for a lithium-ion secondary battery, and thus the amount of thecathode material for a lithium-ion secondary battery packed per unitvolume of the cathode increases. As a result, it is possible to increasethe cathode density, and high-capacity lithium-ion secondary batteriescan be obtained.

The coating ratio of the carbonaceous film with respect to the centralparticles is preferably 60% or more and 95% or less and more preferably80% or more and 95% or less. When the coating ratio of the carbonaceousfilm is 60% or more, the coating effect of the carbonaceous film can besufficiently obtained.

According to the cathode material for a lithium-ion secondary battery ofthe present embodiment, in active material particles including centralparticles represented by Li_(x)A_(y)M_(z)PO₄ (0.95≤x≤1.1, 0.8≤y≤1.1, and0≤z≤0.2; here, A represents at least one element selected from the groupconsisting of Fe, Mn, and Ni, and M represents at least one elementselected from the group consisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al,Ga, In, Si, Ge, and rare earth elements) and a carbonaceous film thatcoats the surfaces of the central particles, when the median diameter is0.50 μm or more and 0.80 μm or less, and the chromaticity b* in theL*a*b* color space is 1.9 or more and 2.3 or less, it is possible toprovide a cathode material for a lithium-ion secondary battery whichsuppresses the peeling of the carbonaceous film that coats the surfacesof the primary particles of a cathode active material and is capable ofimproving the cathode density while guaranteeing the electronconductivity.

Method for Manufacturing Cathode Material for Lithium-Ion SecondaryBattery

The cathode material for a lithium-ion secondary battery of the presentembodiment can be manufactured by cracking active material particlesmade of an agglomerate.

Method for Manufacturing Active Material Particles

A method for manufacturing active material particles in the presentembodiment includes, for example, a manufacturing step of the centralparticles and a precursor of the central particles, a slurry preparationstep of preparing a slurry by mixing at least one central particle rawmaterial selected from the group consisting of the central particles andthe precursor of the central particles, an organic compound which is acarbonaceous film precursor, and water, and a calcination step of dryingthe slurry and calcinating the obtained dried substance in anon-oxidative atmosphere.

Step of Manufacturing Central Particles and Precursor of CentralParticles

As a method for manufacturing a compound (the central particles)represented by Li_(X)A_(y)M_(z)PO₄ (0.95≤x≤1.1, 0.8≤y≤1.1, and 0≤z≤0.2;here, A represents at least one element selected from the groupconsisting of Fe, Mn, and Ni, and M represents at least one elementselected from the group consisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al,Ga, In, Si, Ge, and rare earth elements), a method of the related artsuch as a solid phase method, a liquid phase method, and a gas phasemethod is used. Examples of Li_(x)A_(y)M_(z)PO₄ obtained using theabove-described method include particulate substances (hereinafter, insome cases, referred to as “Li_(x)A_(y)M_(z)PO₄ particles”).

The Li_(x)A_(y)M_(z)PO₄ particles is obtained by, for example,hydrothermally synthesizing a slurry-form mixture obtained by mixing aLi source, an A source, a P source, water, and, as necessary, an Msource. By means of hydrothermal synthesis, Li_(X)A_(y)M_(z)PO₄ isgenerated as a precipitate in water. The obtained precipitate may be aprecursor of Li_(x)A_(y)M_(z)PO₄. In this case, targetLi_(x)A_(y)M_(z)PO₄ particles are obtained by calcinating the precursorof Li_(x)A_(y)M_(z)PO₄.

In this hydrothermal synthesis, a pressure-resistant airtight containeris preferably used.

Here, examples of the Li source include lithium salts such as lithiumacetate (LiCH₃COO) and lithium chloride (LiCl), lithium hydroxide(LiOH), and the like. Among these, as the Li source, at least oneselected from the group consisting of lithium acetate, lithium chloride,and lithium hydroxide is preferably used.

Examples of the A source include chlorides, carboxylates, sulfates, andthe like which include at least one element selected from the groupconsisting of Fe, Mn, and Ni. For example, in a case in which A inLi_(x)A_(y)M_(z)PO₄ is Fe, examples of the Fe source include divalentiron salts such as iron (II) chloride (FeCl₂) iron (II) acetate(Fe(CH₃COO)₂), and iron (II) sulfate (FeSO₄) Among these, as the Fesource, at least one selected from the group consisting of iron (II)chloride, iron (II) acetate, and iron (II) sulfate is preferably used.

Examples of the M source include chlorides, carboxylates, sulfates, andthe like which include at least one element selected from the groupconsisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, andrare earth elements.

Examples of the P source include phosphoric acid compounds such asphosphoric acid (H₃PO₄), ammonium dihydrogen phosphate (NH₄H₂PO₄),diammonium hydrogen phosphate ((NH₄)₂HPO₄), and the like. Among these,as the P source, at least one selected from the group consisting ofphosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogenphosphate is preferably used.

Slurry Preparation Step

By means of the slurry preparation step, the organic compound which isthe precursor of the carbonaceous film is interposed among the centralparticles, and the organic compound and the central particles areuniformly mixed together, and thus it is possible to uniformly coat thesurfaces of the central particles with the organic compound.

Furthermore, by means of the calcination step, the organic compound thatcoats the surfaces of the central particles are carbonized, therebyobtaining active material particles (cathode material) including thecentral particles uniformly coated with the carbonaceous film.

The organic compound that is used in the method for manufacturing activematerial particles in the present embodiment is not particularly limitedas long as the compound is capable of forming the carbonaceous film onthe surfaces of the central particles. Examples of the above-describedorganic compound include divalent alcohols such as polyvinyl alcohol(PVA), polyvinyl pyrrolidone, cellulose, starch, gelatin, carboxymethylcellulose, methyl cellulose, hydroxymethyl cellulose, hydroxyethylcellulose, polyacrylic acid, polystyrene sulfonate, polyacrylamide,polyvinyl acetate, glucose, fructose, galactose, mannose, maltose,sucrose, lactose, glycogen, pectin, alginic acid, glucomannan, chitin,hyaluronic acid, chondroitin, agarose, polyethers, and ethylene glycol,trivalent alcohols such as glycerin, and the like.

In the slurry preparation step, the central particle raw material andthe organic compound are dissolved or dispersed in water, therebypreparing a homogeneous slurry.

When these raw materials are dissolved or dispersed in water, it is alsopossible to add a dispersant thereto.

A method for dissolving or dispersing the central particle raw materialand the organic compound in water is not particularly limited as long asthe central particle raw material is dispersed in water, and the organiccompound is dissolved or dispersed in water. The above-described methodis preferably a method in which a medium stirring-type dispersion devicethat stirs medium particles at a high rate such as a planetary ballmill, an oscillation ball mill, a bead mill, a paint shaker, or anattritor is used.

When the central particle raw material and the organic compound aredissolved or dispersed in water, it is preferable to disperse thecentral particle raw material in water in a primary particle form, then,add the organic compound to water, and stir the organic compound so asto be dissolved or dispersed. In such a case, the surfaces of theprimary particles of the central particle raw material are easily coatedwith the organic compound. Therefore, the organic compound is uniformlydispersed on the surfaces of the primary particles of the centralparticle raw material, and consequently, the surfaces of the primaryparticles of the central particles are coated with the carbonaceous filmderived from the organic compound.

Calcination Step

Next, the slurry prepared in the slurry preparation step is sprayed anddried in a high-temperature atmosphere, for example, in the atmosphereof 70° C. or higher and 250° C. or lower.

Next, the obtained dried substance is calcinated in a non-oxidativeatmosphere at a temperature of preferably 500° C. or higher and 1,000°C. or lower and more preferably 600° C. or higher and 1,000° C. or lowerfor 0.1 hours or longer and 40 hours or shorter.

The non-oxidative atmosphere is preferably an atmosphere filled with aninert gas such as nitrogen (N₂), argon (Ar), or the like. In a case inwhich it is necessary to further suppress the oxidation of the driedsubstance, a reducing atmosphere including approximately severalpercentages by volume of a reducing gas such as hydrogen (H₂) ispreferred. In addition, for the purpose of removing organic componentsevaporated in the non-oxidative atmosphere during calcination, asusceptible or burnable gas such as oxygen (O₂) may be introduced intothe non-oxidative atmosphere.

Here, when the calcination temperature is set to 500° C. or higher, itis easy for the organic compound in the dried substance to besufficiently decomposed and reacted, and the organic compound is easilyand sufficiently carbonized. As a result, it is easy to prevent thegeneration of a high-resistance decomposed substance of the organiccompound in the obtained agglomerate. Meanwhile, when the calcinationtemperature is set to 1,000° C. or lower, lithium (Li) in the centralparticle raw material is not easily evaporated, and the grain growth ofthe central particles to a size that is equal to or larger than thetarget size is suppressed. As a result, in a case in which a lithium-ionsecondary battery including a cathode including the cathode material ofthe present embodiment is produced, it is possible to prevent thedischarge capacity at a high charge-discharge rate from decreasing, andit is possible to realize lithium-ion secondary batteries havingsufficient charge and discharge rate performance.

By means of the above-described steps, active material particles made ofan agglomerate in which the surfaces of the primary particles of thecentral particles are coated with carbon (the carbonaceous film)generated by the thermal decomposition of the organic compound in thedried substance are obtained.

Cracking step of active material particles Next, at least part of theactive material particles made of this agglomerate are cracked. Here, inorder to “crack at least part of the active material particles made ofthe agglomerate”, at least part of the agglomerate needs to be cracked,and not all the agglomerate needs to be cracked.

A device that is used for the cracking of the agglomerate needs to becapable of cracking not all of the agglomerate but part of theagglomerate, and, for example, an air flow-type fine crusher such as adry-type ball mill, a wet-type ball mill, a mixer, or a jet mill, anultrasonic crusher, or the like is used.

In the present embodiment, a jet mill is preferably used for crackingsince the damage of the active material particles (the central particlesand the primary particles) is suppressed.

In addition, the supply rate of the agglomerate into the jet mill ispreferably set to 50 g/hour to 1,500 g/hour, and the air pressure ispreferably set to 0.3 MPa to 0.7 MPa. The cracking intensity can befreely adjusted by varying the supply rate of the agglomerate beinginjected into the jet mill. In addition, the median diameter and thechromaticity b* of the cathode material for a lithium-ion secondarybattery can be adjusted by adjusting the cracking intensity. Here, in acase in which the cracking intensity is weak, the median diameter islarge, and the chromaticity b* becomes a low value. On the other hand,in a case in which the cracking intensity is strong, the median diameteris small, and the chromaticity b* becomes a high value. When thechromaticity b* is a high value, the degree of exposure of the centralparticles that are not coated with the carbonaceous film increases inthe cathode material for a lithium-ion secondary battery.

Cathode for Lithium-Ion Secondary Battery

A cathode for a lithium-ion secondary battery of the present embodiment(hereinafter, in some cases, referred to as “cathode”) includes thecathode material for a lithium-ion secondary battery of the presentembodiment. In more detail, the cathode of the present embodimentincludes an electrode current collector made of a metal foil and acathode mixture layer formed on the electrode current collector, and thecathode mixture layer includes the cathode material for a lithium-ionsecondary battery of the present embodiment. That is, the cathode of thepresent embodiment is obtained by forming a cathode mixture layer on onemain surface of the electrode current collector using the cathodematerial for a lithium-ion secondary battery of the present embodiment.

The cathode of the present embodiment is mainly used as a cathode for alithium-ion secondary battery.

Since the cathode for a lithium-ion secondary battery of the presentembodiment includes the cathode material for a lithium-ion secondarybattery of the present embodiment, lithium-ion secondary batteries forwhich the cathode for a lithium-ion secondary battery of the presentembodiment is used have a high energy density and have excellent inputand output characteristics.

Method for Manufacturing Cathode for Lithium-Ion Secondary Battery

The method for manufacturing the cathode for a lithium-ion secondarybattery of the present embodiment is not particularly limited as long asa cathode mixture layer can be formed on one main surface of a currentcollector using the cathode material for a lithium-ion secondary batteryof the present embodiment. Examples of the method for manufacturing thecathode of the present embodiment include the following method.

First, the cathode material for a lithium-ion secondary battery of thepresent embodiment, a binding agent made of a binder resin, and asolvent are mixed together, thereby preparing cathode material paste. Atthis time, to the cathode material paste in the present embodiment, aconductive auxiliary agent such as carbon black may be added thereto ifnecessary.

Binding Agent

As the binding agent, that is, the binder resin, for example, apolytetrafluoroethylene (PTFE) resin, a polyvinylidene fluoride (PVdF)resin, fluorine rubber, or the like is preferably used.

The blending amount of the binding agent used to prepare the cathodematerial paste is not particularly limited and is, for example,preferably 1 part by mass or more and 30 parts by mass or less and morepreferably 3 parts by mass or more and 20 parts by mass or less withrespect to 100 parts by mass of the cathode material for a lithium-ionsecondary battery.

When the blending amount of the binding agent is 1 part by mass or more,it is possible to sufficiently increase the binding property between thecathode mixture layer and the electrode current collector. Therefore, itis possible to prevent the cathode mixture layer from being cracked ordropped during a molding of the cathode mixture layer by means ofrolling or the like. In addition, it is possible to prevent the cathodemixture layer from being peeled off from the electrode current collectorin a process of charging and discharging lithium-ion secondary batteriesand prevent the battery capacity or the charge-discharge rate from beingdecreased. On the other hand, when the amount of the binding agentblended is 30 parts by mass or less, it is possible to prevent theinternal resistance of the cathode material for a lithium-ion secondarybattery from being decreased and prevent the battery capacity at a highcharge-discharge rate from being decreased.

Conductive Auxiliary Agent

The conductive auxiliary agent is not particularly limited, and, forexample, at least one element selected from the group consisting offibrous carbon such as acetylene black (AB), KETJEN BLACK, furnaceblack, vapor-grown carbon fiber (VGCF), and carbon nanotube is used.

Solvent

The solvent that is used in the cathode material paste including thecathode material for a lithium-ion secondary battery of the presentembodiment is appropriately selected depending on the properties of thebinding agent. When the solvent is appropriately selected, it ispossible to facilitate the cathode material paste to be applied tosubstances to be coated such as electrode current collectors.

Examples of the solvent include water, alcohols such as methanol,ethanol, 1-propanol, 2-propanol (isopropyl alcohol: IPA), butanol,pentanol, hexanol, octanol, and diacetone alcohol, esters such as ethylacetate, butyl acetate, ethyl lactate, propylene glycol monomethyl etheracetate, propylene glycol monoethyl ether acetate, and γ-butyrolactone,ethers such as diethyl ether, ethylene glycol monomethyl ether (methylcellosolve), ethylene glycolmonoethylether (ethyl cellosolve), ethyleneglycol monobutyl ether (butyl cellosolve), diethylene glycol monomethylether, and diehtylene glycol monoethyl ether, ketones such as acetone,methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), acetylacetone,and cyclohexanone, amides such as dimethyl formamide,N,N-dimethylacetoacetamide, and N-methyl-2-pyrrolidone (NMP), glycolssuch as ethylene glycol, diethylene glycol, and propylene glycol, andthe like. These solvents may be used singly, or a mixture of two or moresolvents may be used.

The content rate of the solvent in the cathode material paste ispreferably 50% by mass or more and 70% by mass or less and morepreferably 55% by mass or more and 65% by mass or less in a case inwhich the total mass of the cathode material for a lithium-ion secondarybattery of the present embodiment, the binding agent, and the solvent isset to 100 parts by mass.

When the content rate of the solvent in the cathode material paste is inthe above-described range, it is possible to obtain cathode materialpaste having excellent cathode formability and excellent batterycharacteristics.

A method for mixing the cathode material for a lithium-ion secondarybattery of the present embodiment, the binding agent, the conductiveauxiliary agent, and the solvent is not particularly limited as long asthese components can be uniformly mixed together. Examples thereofinclude mixing methods in which a kneader such as a ball mill, a sandmill, a planetary (sun-and-planet) mixer, a paint shaker, or ahomogenizer is used.

The cathode material paste is applied to one main surface of theelectrode current collector so as to form a coated film, and then thiscoated film is dried, thereby obtaining the electrode current collectorhaving a coated film made of a mixture of the cathode material and thebinding agent formed on one main surface.

After that, the coated film is pressed by pressure and is dried, therebyproducing a cathode having a cathode mixture layer on one main surfaceof the electrode current collector.

Lithium-Ion Secondary Battery

A lithium-ion secondary battery of the present embodiment includes acathode, an anode, and a non-aqueous electrolyte, in which the cathodeis the cathode for a lithium-ion secondary battery of the presentembodiment. Specifically, the lithium-ion secondary battery of thepresent embodiment includes the cathode for a lithium-ion secondarybattery of the present embodiment as a cathode, an anode, a separator,and a non-aqueous electrolyte.

In the lithium-ion secondary battery of the present embodiment, theanode, the non-aqueous electrolyte, and the separator are notparticularly limited.

Anode

Examples of the anode include anodes including an anode material such asLi metal, carbon materials such as natural graphite and hard carbon, Lialloys, Li₄Ti₅O₁₂, Si(Li_(4.4)Si), and the like.

Non-Aqueous Electrolyte

Examples of the non-aqueous electrolyte include non-aqueous electrolytesobtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate(EMC) so that the volume ratio reaches 1:1 and dissolving lithiumhexafluorophosphate (LiPF₆) in the obtained solvent mixture so that theconcentration reaches 1 mol/dm³.

Separator

As the separator, it is possible to use, for example, porous propylene.

In addition, instead of the non-aqueous electrolyte and the separator, asolid electrolyte may be used.

Since the lithium-ion secondary battery of the present embodimentincludes the cathode for a lithium-ion secondary battery of the presentembodiment as the cathode, the lithium-ion secondary battery has a highenergy density and has excellent input and output characteristics.

EXAMPLES

Hereinafter, the present invention will be more specifically describedusing examples and comparative examples, but the present invention isnot limited- to the following examples.

Example 1

Synthesis of Cathode Material for Lithium-Ion Secondary Battery

Lithium phosphate (Li₃PO₄) (2 mol) and iron (II) sulfate (FeSO₄) (2 mol)were added to and mixed with water so that the total amount reached 4 L,thereby preparing a homogeneous slurry-form mixture.

Next, this mixture was stored in a pressure-resistant airtight containerhaving a capacity of 8 L and was hydrothermally synthesized at 200° C.for four hours, thereby generating a precipitate.

Next, this precipitate was cleaned with water, thereby obtaining acake-form precursor of a cathode active material.

Next, a polyethylene glycol (20 g) as the organic compound and zirconiaballs having a diameter of 5 mm as medium particles were used in theprecursor of the cathode active material (150 g in terms of solidcontents), and a dispersion treatment was performed in a ball mill fortwo hours, thereby preparing a homogeneous slurry.

Next, this slurry was sprayed in the atmosphere at 200° C. and dried,thereby obtaining an agglomerate of the precursor of the cathodematerial which was coated with an organic substance having an averageparticle diameter of 8.5 μm.

Next, the obtained dried powder was calcinated in a nitrogen atmospherefor three hours at 700° C., thereby obtaining an agglomerate having anaverage particle diameter of 8.5 μm.

Cracking of Agglomerate

The above-described agglomerate was cracked using a jet mill device(manufactured by Nisshin Engineering Inc., trade name: SJ-100) so thatthe median diameter reached 0.74 μm, thereby obtaining a cathodematerial 1 of Example 1.

Production of Lithium-Ion Secondary Battery

The cathode material 1, polyvinylidene fluoride (PVdF) as a bindingagent, and acetylene black (AB) as a conductive auxiliary agent wereadded to N-methyl-2-pyrrolidone (NMP) which is a solvent so that themass ratio (the cathode material 1:AB:PVdF) in paste reached 94:1:5, andthe components were mixed together and kneaded using a kneader(manufactured by Thinky Corporation, trade name: AWATORI RENTARO) for 30minutes under conditions of a revolution rate of 1,200 rpm and arotation rate of 800 rpm, thereby preparing cathode material paste (forthe cathode).

This cathode material paste (for the cathode) was applied onto thesurface of a 30 μm-thick aluminum foil (electrode current collector) soas to form a coated film, and the coated film was dried, thereby forminga cathode mixture layer on the surface of the aluminum foil.

After that, the cathode mixture layer was pressed at a pressure of 58.84MPa, thereby producing a cathode 1 of Example 1.

A lithium metal was disposed as an anode with respect to this cathode 1,and a separator made of porous polypropylene was disposed between thecathode 1 and the anode, there by producing a member for a battery 1.

Meanwhile, ethylene carbonate and diethyl carbonate were mixed togetherin a mass volume of 1:1, and furthermore, 1 M of a LiPF₆ solution wasadded thereto, thereby producing an electrolyte solution 1 havinglithium ion conductivity.

Next, the member for a battery 1 was immersed in the electrolytesolution 1, there by producing a lithium-ion secondary battery 1 ofExample 1.

Example 2

A cathode material 2 of Example 2 was obtained in the same manner as inExample 1 except for the fact that the cracking intensity of the jetmill device was set to 1.41 of the intensity in Example 1.

A lithium-ion secondary battery 2 of Example 2 was produced in the samemanner as in Example 1 except for the fact that the cathode material 2was used.

Example 3

A cathode material 3 of Example 3 was obtained in the same manner as inExample 1 except for the fact that the cracking intensity of the jetmill device was set to 2.50 of the intensity in Example 1.

A lithium-ion secondary battery 3 of Example 3 was produced in the samemanner as in Example 1 except for the fact that the cathode material 3was used.

Example 4

A cathode material 4 of Example 4 was obtained in the same manner as inExample 1 except for the fact that the cracking intensity of the jetmill device was set to 3.75 of the intensity in Example 1.

A lithium-ion secondary battery 4 of Example 4 was produced in the samemanner as in Example 1 except for the fact that the cathode material 4was used.

Example 5

A cathode material 5 of Example 5 was obtained in the same manner as inExample 1 except for the fact that the cracking intensity of the jetmill device was set to 4.50 of the intensity in Example 1.

A lithium-ion secondary battery 5 of Example 5 was produced in the samemanner as in Example 1 except for the fact that the cathode material 5was used.

Example 6

A cathode material 6 of Example 6 was obtained in the same manner as inExample 1 except for the fact that the cracking intensity of the jetmill device was set to 5.26 of the intensity in Example 1.

A lithium-ion secondary battery 6 of Example 6 was produced in the samemanner as in Example 1 except for the fact that the cathode material 6was used.

Example 7

Synthesis of cathode material for lithium-ion secondary battery Lithiumphosphate (Li₃PO₄) (2 mol) and iron (II) sulfate (FeSO₄) (2 mol) wereadded to and mixed with water so that the total amount reached 4 L,thereby preparing a homogeneous slurry-form mixture.

Next, this mixture was stored in a pressure-resistant airtight containerhaving a capacity of 8 L and was hydrothermally synthesized at 200° C.for 24 hours, thereby generating a precipitate.

Next, this precipitate was cleaned with water, thereby obtaining acake-form precursor of a cathode active material.

Next, a polyethylene glycol (20 g) as the organic compound and zirconiaballs having a diameter of 5 mm as medium particles were used in theprecursor of the cathode active material (150 g in terms of solidcontents), and a dispersion treatment was performed in a ball mill fortwo hours, thereby preparing a homogeneous slurry.

Next, this slurry was sprayed in the atmosphere at 200° C. and dried,thereby obtaining an agglomerate of the precursor of the cathodematerial which was coated with an organic substance having an averageparticle diameter of 8.3 μm.

Next, the obtained dried powder was calcinated in a nitrogen atmospherefor three hours at 700° C., thereby obtaining an agglomerate having anaverage particle diameter of 8.3 μm.

Cracking of Agglomerate

The above-described agglomerate was cracked using a jet mill device(manufactured by Nisshin Engineering Inc., trade name: SJ-100) so thatthe cracking intensity reached 4.50 of the cracking intensity in Example1, thereby obtaining a cathode material 7 of Example 7.

A lithium-ion secondary battery 7 of Example 7 was produced in the samemanner as in Example 1 except for the fact that the cathode material 7was used.

Example 8

Synthesis of Cathode Material for Lithium-Ion Secondary Battery

Lithium phosphate (Li₃PO₄) (2 mol) and iron (II) sulfate (FeSO₄) (2 mol)were added to and mixed with water so that the total amount reached 4 L,thereby preparing a homogeneous slurry-form mixture.

Next, this mixture was stored in a pressure-resistant airtight containerhaving a capacity of 8 L and was hydrothermally synthesized at 160° C.for two hours, thereby generating a precipitate.

Next, this precipitate was cleaned with water, thereby obtaining acake-form precursor of a cathode active material.

Next, a polyethylene glycol (20 g) as the organic compound and zirconiaballs having a diameter of 5 mm as medium particles were used in theprecursor of the cathode active material (150 g in terms of solidcontents), and a dispersion treatment was performed in a ball mill fortwo hours, thereby preparing a homogeneous slurry.

Next, this slurry was sprayed in the atmosphere at 200° C. and dried,thereby obtaining an agglomerate of the precursor of the cathodematerial which was coated with an organic substance having an averageparticle diameter of 8.9 μm.

Next, the obtained dried powder was calcinated in a nitrogen atmospherefor three hours at 700° C., thereby obtaining an agglomerate having anaverage particle diameter of 8.9 μm.

Cracking of Agglomerate

The above-described agglomerate was cracked using a jet mill device(manufactured by Nisshin Engineering Inc., trade name: SJ-100) so thatthe cracking intensity reached 5.64 of the cracking intensity in Example1, thereby obtaining a cathode material 8 of Example 8.

A lithium-ion secondary battery 8 of Example 8 was produced in the samemanner as in Example 1 except for the fact that the cathode material 8was used.

Example 9

Synthesis of Cathode Material for Lithium-Ion Secondary Battery

Lithium phosphate (Li₃PO₄) (2 mol) and iron (II) sulfate (FeSO₄) (2 mol)were added to and mixed with water so that the total amount reached 4 L,thereby preparing a homogeneous slurry-form mixture.

Next, this mixture was stored in a pressure-resistant airtight containerhaving a capacity of 8 L and was hydrothermally synthesized at 120° C.for five hours, thereby generating a precipitate.

Next, this precipitate was cleaned with water, thereby obtaining acake-form precursor of a cathode active material.

Next, a polyethylene glycol (20 g) as the organic compound and zirconiaballs having a diameter of 5 mm as medium particles were used in theprecursor of the cathode active material (150 g in terms of solidcontents), and a dispersion treatment was performed in a ball mill fortwo hours, thereby preparing a homogeneous slurry.

Next, this slurry was sprayed in the atmosphere at 200° C. and dried,thereby obtaining an agglomerate of the precursor of the cathodematerial which was coated with an organic substance having an averageparticle diameter of 9.1 μm.

Next, the obtained dried powder was calcinated in a nitrogen atmospherefor three hours at 700° C., thereby obtaining an agglomerate having anaverage particle diameter of 9.1 μm.

Cracking of Agglomerate

The above-described agglomerate was cracked using a jet mill device(manufactured by Nisshin Engineering Inc., trade name: SJ-100) so thatthe cracking intensity reached 6.00 of the cracking intensity in Example1, thereby obtaining a cathode material 9 of Example 9.

A lithium-ion secondary battery 9 of Example 9 was produced in the samemanner as in Example 1 except for the fact that the cathode material 9was used.

Comparative Example 1

A cathode material 10 of Comparative Example 1 was obtained in the samemanner as in Example 1 except for the fact that the cracking intensityof the jet mill device was set to 0.69 of the intensity in Example 1.

A lithium-ion secondary battery 10 of Comparative Example 1 was producedin the same manner as in Example 1 except for the fact that the cathodematerial 10 was used.

Comparative Example 2

A cathode material 11 of Comparative Example 2 was obtained in the samemanner as in Example 1 except for the fact that the cracking intensityof the jet mill device was set to 0.90 of the intensity in Example 1.

A lithium-ion secondary battery 11 of Comparative Example 2 was producedin the same manner as in Example 1 except for the fact that the cathodematerial 11 was used.

Comparative Example 3

A cathode material 12 of Comparative Example 3 was obtained in the samemanner as in Example 1 except for the fact that the cracking intensityof the jet mill device was set to 5.64 of the intensity in Example 1.

A lithium-ion secondary battery 12 of Comparative Example 3 was producedin the same manner as in Example 1 except for the fact that the cathodematerial 12 was used.

Comparative Example 4

A cathode material 13 of Comparative Example 4 was obtained in the samemanner as in Example 1 except for the fact that the cracking intensityof the jet mill device was set to 6.43 of the intensity in Example 1.

A lithium-ion secondary battery 13 of Comparative Example 4 was producedin the same manner as in Example 1 except for the fact that the cathodematerial 13 was used.

Comparative Example 5

A cathode material 14 of Comparative Example 5 was obtained in the samemanner as in Example 8 except for the fact that the cracking intensityof the jet mill device was set to 6.00 of the intensity in Example 1.

A lithium-ion secondary battery 14 of Comparative Example 5 was producedin the same manner as in Example 1 except for the fact that the cathodematerial 14 was used.

Comparative Example 6

A cathode material 15 of Comparative Example 6 was obtained in the samemanner as in Example 1 except for the fact that the cracking intensityof the jet mill device was set to 6.43 of the intensity in Example 9.

A lithium-ion secondary battery 15 of Comparative Example 6 was producedin the same manner as in Example 1 except for the fact that the cathodematerial 15 was used.

Evaluation of Cathode Material for Lithium-Ion Secondary Battery andLithium-Ion Secondary Battery

The cathodes material for a lithium-ion secondary battery and thelithium-ion secondary batteries of Examples 1 to 9 and ComparativeExamples 1 to 6 were evaluated as described below.

1. Specific Surface Area

The BET specific surface area of the cathode material for a lithium-ionsecondary battery was measured using a measurement device (manufacturedby Mountech Co., Ltd., trade name: HM model-1208) and a one-point methodat a relative pressure of 0.29 (P/P₀).

2. Amount of Carbon

The amount of carbon in the cathode material for a lithium-ion secondarybattery was measured using a carbon/sulfur analyzer (manufactured byHoriba Ltd., trade name: EMIA-220V).

3. Median Diameter

The median diameter in the cathode material for a lithium-ion secondarybattery was measured using the following method.

The median diameter was measured using a measurement device(manufactured by Horiba Ltd., trade name: LA-950V2).

First, pure water (40 g) and polyvinylpyrrolidone (PVP) (0.12 g) asdispersion liquids and the cathode material for a lithium-ion secondarybattery (0.04 g) as specimen powder were weighed in a 70 mL mayonnaisebottle. This mayonnaise bottle was manually shaken approximately tentimes so as to mix the specimen powder and the dispersion liquids well.

Next, the mixed solution of the specimen powder and the dispersionliquids was treated with ultrasonic waves for two minutes underconditions of output 5 and pulse 50% in an ultrasonic homogenizer(manufactured by Branson Ultrasonics, Emersion Japan, Ltd., trade name:SONIFIER450), and the median diameter was measured using the obtaineddispersion solution.

The median diameter was measured with the data loading number set to5,000 in terms of LD and 1,000 in terms of LED, and the data computationconditions were as described below.

Computation Conditions

(Sample Refractive Index)

LD real part: 1.48

LD imaginary part: 0.45

LED real part: 1.50

LED imaginary part: 0.55

Dispersion Medium Refractive Index

LD real part: 1.33

LD imaginary part: 0.00

LED real part: 1.33

LED imaginary part: 0.00

(Number of repetitions): 15 times

(Particle diameter criterion): Volume

(Computation algorithm): Standard computation

4. Chromaticity b*

The chromaticity b* in the L*a*b* color space of the cathode materialfor a lithium-ion secondary battery was obtained by means of reflectedlight two-degree visual field measurement in which a spectroscopiccalorimeter (Serial No.: SE-2000, manufactured by Nippon DenshokuIndustries Co., Ltd.) and a D65 light source. When the chromaticity b*of the cathode material for a lithium-ion secondary battery wasmeasured, the cathode material of the measurement subject was evenlyplaced on a scale, and the chromaticity b* of the cathode material wasmeasured.

5. Cathode Density

The cathode density of the cathode material for a lithium-ion secondarybattery was computed as a ratio of the mass of the cathode material asthe numerator to the volume of the cathode which was the compressedcathode excluding the aluminum electrode current collector as thedenominator.

In addition, in a case in which the cathode density was 2.00 g/cc ormore, the cathode density was evaluated as A, in a case in which thecathode density was 1.90 g/cc or more, the cathode density was evaluatedas B, and, in a case in which the cathode density was less than 1.90g/cc, the cathode density was evaluated as C.

6. Capacity Retention after 20 Cycles

Regarding the capacity retention of the lithium-ion secondary batteryafter 20 cycles, when constant-current charging at a current value of0.25 C until the battery voltage reached 3.7 V and then discharging at acurrent value of 0.5 C until the battery voltage reached 2.5 V in anenvironment of 45° C. was considered as one cycle, and this cycle wasrepeated 20 times, the fraction of the discharge capacity at the 20^(th)cycle as the numerator to the discharge capacity at the first cycle asthe denominator was evaluated as the capacity retention. In a case inwhich the electron conductivity of the cathode material is notsufficiently guaranteed, active material particles repeatedly expand andshrink in accordance with the charge and discharge cycle, and thus thenumber of electron conduction paths in the cathode becomes insufficient,and thus the capacity retention decreases.

In addition, in a case in which the capacity retention after 20 cycleswas 70% or more, the electron conductivity was evaluated as B, and, in acase in which the capacity retention after 20 cycles was less than 70%,the electron conductivity was evaluated as C.

Evaluation Results

The evaluation results of the cathode materials for a lithium-ionsecondary battery and the lithium-ion secondary batteries of Examples 1to 9 and Comparative Examples 1 to 6 are shown in Table 1. Meanwhile,the amount of carbon in Table 1 is the amount (parts by mass) of carbonthat formed the carbonaceous film with respect to 100 parts by mass ofthe cathode active material.

TABLE 1 Amount Capacity Cracking Specific of retention intensity surfacecarbon Median Cathode Cathode after 20 Electron (relative area [partsChromaticity diameter density density cycles conductivity ratio) [m²/g]by mass] b* [μm] [g/cc] evaluation [%] evaluation Example 1 1.00 8.41.05 1.90 0.78 1.92 B 74 B Example 2 1.41 8.4 1.05 1.96 0.73 1.93 B 75 BExample 3 2.50 8.5 1.05 2.00 0.70 1.98 B 75 B Example 4 3.75 8.5 1.052.13 0.65 1.97 B 75 B Example 5 4.50 8.6 1.05 2.24 0.60 1.99 B 73 BExample 6 5.26 8.7 1.05 2.28 0.56 2.02 A 74 B Example 7 4.50 5.6 0.682.27 0.52 2.04 A 78 B Example 8 5.64 15.3 1.85 2.28 0.61 1.97 B 73 BExample 9 6.00 24.6 2.90 2.29 0.69 1.93 B 72 B Comparative 0.69 8.3 1.051.85 0.84 1.79 C 76 B Example 1 Comparative 0.90 8.3 1.05 1.87 0.81 1.87C 74 B Example 2 Comparative 5.64 8.9 1.05 2.33 0.54 2.03 A 62 C Example3 Comparative 6.43 9.2 1.05 2.41 0.48 2.05 A 51 C Example 4 Comparative6.00 15.3 1.85 2.32 0.57 2.03 A 60 C Example 5 Comparative 6.43 24.62.90 2.34 0.61 2.01 A 59 C Example 6

When Examples 1 to 6 and Comparative Examples 1 to 4 are compared usingthe results of Table 1, in Comparative Examples 1 and 2 in which thechromaticity b* was 1.9 or less and the median diameter exceeded 0.80μm, the cracking was not sufficient, and thus the cathode density was aslow as 1.9 g/cc or less, and the energy density per unit volume was poorwhen used as a cathode material for a lithium-ion secondary battery.

In addition, in Comparative Example 4 in which the chromaticity b* was2.3 or more and the median diameter was less than 0.50 μm, the crackingintensity was too high, and thus the carbonaceous film that coated thesurfaces of the central particles was peeled off during the cracking,the capacity retention after 20 cycles was as lows as 70% or less, andthe input and output characteristics and the durability were poor whenused as a cathode material for a lithium-ion secondary battery.

On the other hand, in Examples 1 to 6 in which the median diameter was0.50 μm or more and 0.80 μm or less and the chromaticity b* was 1.9 ormore and 2.3 or less, excellent battery characteristics of the cathodedensity being 1.9 g/cc or more and the capacity retention after 20cycles being 70% or more was exhibited, and it is possible to providelithium-ion secondary batteries having excellent energy density, inputand output characteristics, and durability when used as a cathodematerial for a lithium-ion secondary battery.

In addition, when Examples 6 to 9 and Comparative Examples 5 and 6 werecompared, it is also possible to confirm that, even when the specificsurface area and the amount of carbon vary, if the cracking intensity isadjusted so that the median diameter is 0.50 μm or more and 0.80 μm orless and the chromaticity b* is 1.9 or more and 2.3 or less, it ispossible to develop excellent battery characteristics of the cathodedensity reaching 1.9 g/cc or more and the capacity retention after 20cycles reaching 70% or more.

Lithium-ion secondary batteries for which the cathode material for alithium-ion secondary battery of the present invention is used haveexcellent energy density, input and output characteristics, anddurability and are thus capable of significantly contributing to theadvancement of the reliability of lithium-ion secondary batteriescommencing with mobile body applications.

What is claimed is:
 1. A cathode material for a lithium-ion secondarybattery which is active material particles including central particlesrepresented by Li_(x)A_(y)M_(z)PO₄, wherein 0.95≤x≤1.1, 0.8≤y≤1.1, and0≤z≤0.2, A represents at least one element selected from the groupconsisting of Fe, Mn, and Ni, and M represents at least one elementselected from the group consisting of Mg, Ca, Co, Sr, Ba, Ti, Zn, B, Al,Ga, In, Si, Ge, and rare earth elements, and a carbonaceous film thatcoats surfaces of the central particles, wherein the active materialparticles is a crushed powder obtained by crushing an aggregate of theactive material particles by using a jet mill under a condition in whicha supply rate of the aggregate into the jet mill is set to 50 g/hour to1,500 g/hour, and an air pressure is set to 0.3 MPa to 0.7 MPa, a mediandiameter of the crushed powder particles is 0.52 μm or more and 0.78 μmor less, a chromaticity b* in an L*a*b* color space of the activematerial particles is 1.9 or more and 2.3 or less, and a thickness ofthe carbonaceous film is 0.2 nm or more and 10 nm or less, a coatingratio of the carbonaceous film with respect to the central particles is60% or more and 95% or less, the average primary particle diameter ofthe primary particles of the central particles is 20 nm or more and 500nm or less.
 2. The cathode material for a lithium-ion secondary batteryaccording to claim 1, wherein the central particles are LiFePO₄.
 3. Thecathode material for a lithium-ion secondary battery according to claim1, wherein the median diameter is 0.55 μm or more and 0.75 μm or less,and the chromaticity b* is 1.95 or more and 2.3 or less.
 4. The cathodematerial for a lithium-ion secondary battery according to claim 1,wherein a BET specific surface area is 5 m²/g or more and 25 m²/g orless, and an amount of carbon forming the carbonaceous film is 0.1 partsby mass or more and 10 parts by mass or less with respect to 100 partsby mass of the central particles.
 5. A cathode for a lithium-ionsecondary battery, comprising: an electrode current collector; and acathode mixture layer formed on the electrode current collector, whereinthe cathode mixture layer includes the cathode material for alithium-ion secondary battery according to claim
 1. 6. A lithium-ionsecondary battery comprising: the cathode for a lithium-ion secondarybattery according to claim
 5. 7. The cathode material for a lithium-ionsecondary battery according to claim 1, wherein the crushed powder is asecondary particle of the active material particles.
 8. The cathodematerial for a lithium-ion secondary battery according to claim 1,wherein the median diameter of the crushed powder is measured by a laserdiffraction, the average primary particle diameter of the primaryparticles of the central particles is measured by a scanning electronmicroscope.
 9. The cathode material for a lithium-ion secondary batteryaccording to claim 1, wherein the amount of carbon forming thecarbonaceous film is 0.1 parts by mass or more and 3 parts by mass orless with respect to 100 parts by mass of the central particles.
 10. Thecathode material for a lithium-ion secondary battery according to claim1, wherein a carbon amount of the primary particles with respect to aspecific surface area of the primary particles represented by [thecarbon amount]/[the specific surface area of the primary particles ofthe central particles] is 0.01 g/m2 or more and 0.5 g/m2 or less. 11.The cathode material for a lithium-ion secondary battery according toclaim 1, wherein the carbonaceous film is formed by carbonizingpolyethylene glycol.
 12. The cathode material for a lithium-ionsecondary battery according to claim 1, wherein an amount of carbonforming the carbonaceous film is 0.1 to 2.90 parts by mass with respectto 100 parts by mass of the central particles.
 13. The cathode materialfor a lithium-ion secondary battery according to claim 1, wherein anamount of carbon forming the carbonaceous film is 0.1 to 1.85 parts bymass with respect to 100 parts by mass of the central particles.